Around this time last year, NASA’s Hubble Space Telescope spotted evidence of water vapor venting off the south polar region of Jupiter’s moon Europa. Since previous work suggested that an ocean existed underneath its icy crust, astronomers thought that water plumes were erupting off Europa’s surface (pictured above), reaching altitudes of up to 200 kilometers. Then in September of this year, astronomers announced that the giant geysers have mysteriously vanished. Now, several teams searching for these plumes report that they’ve come up empty too.
First, researchers who’ve trained Hubble on Europa repeatedly over several months this year have failed to confirm the plumes. "We have not yet found any signals of water vapor in the new images so far," Southwest Research Institute’s Lorenz Roth said at the fall meeting of the American Geophysical Union (AGU) in San Francisco this month, according to Space.com.
Furthermore, researchers analyzing data from Cassini’s 2001 Jupiter flyby report that they see no evidence of plume activity in the thin, hot gas around the moon. And if there is plume activity, it’s most likely intermittent.
As Cassini sped through Jupiter’s system on its way to Saturn, the spacecraft’s ultraviolet imaging spectrograph (UVIS) showed that the hot, excited gas (or plasma) around Europa originates not from the moon itself but from volcanoes on the nearby moon Io. Additionally, Europa contributes 40 times less oxygen to its surrounding environment than researchers previously thought, and its tenuous atmosphere -- which is already millions of times thinner than Earth’s -- is about 100 times less dense than older estimates. This new downward revision in the amount of oxygen Europa pumps into the environment around Jupiter makes it less likely that plumes of water vapor are regularly being vented high into orbit. And plasma this hot further suggests that Europa isn’t outputting large amounts of gas, including water vapor.
Cassini has, however, detected ongoing plume activity at Saturn’s moon Enceladus starting in 2005. "It is certainly still possible that plume activity occurs, but that it is infrequent or the plumes are smaller than we see at Enceladus," Planetary Science Institute’s Amanda Hendrix says in a NASA release.
Another team reanalyzed images gathered by NASA's Galileo probe -- which studied the Jovian system up close from 1995 to 2003 -- and also turned up no evidence for the plumes. "I find it hard to believe that if a plume that was similar to the plumes we see on Enceladus had been going off on Europa during the Galileo era -- I find it really unlikely that we would have missed it," Cynthia Phillips of the SETI said at AGU, according to Space.com. "I think we would have seen that thing."
The hunt continues.